Use ART_TARGET_ANDROID instead of __ANDROID__

On golem, we build for ART_TARGET_LINUX, which doesn't have support for
linkerconfig. Therefore, guard all linker namespace code on
ART_TARGET_ANDROID instead of __ANDROID__ (which is unfortunately not
under ART's control).

Test: build for golem and run
Bug: 154074847
Change-Id: I7a2b81918177704b42d8aafbd6d7e9d06d34e5f4
diff --git a/libnativeloader/library_namespaces.cpp b/libnativeloader/library_namespaces.cpp
index 6092c00..d42a4b5 100644
--- a/libnativeloader/library_namespaces.cpp
+++ b/libnativeloader/library_namespaces.cpp
@@ -13,6 +13,9 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
+#if defined(ART_TARGET_ANDROID)
+
 #include "library_namespaces.h"
 
 #include <dirent.h>
@@ -392,3 +395,5 @@
 }
 
 }  // namespace android::nativeloader
+
+#endif  // defined(ART_TARGET_ANDROID)